In a general way this explains the respiratory changes that occur in the blood in the lungs.  Blood containing oxygen and carbon dioxid is flowing in countless tiny streams through the walls of the air cells of the lungs.  The air cells themselves contain a mixture of the same two gases.  A thin, moist membrane, well adapted to allow gaseous diffusion, separates the blood from the air.  This membrane is the delicate wall of the capillaries and the epithelium of the air cells.  By experiment it has been found that the pressure of oxygen in the blood is less than that in the air cells, and that the pressure of carbon dioxid gas in the blood is greater than that in the air cells.  As a result, a diffusion of gases ensues.  The blood gains oxygen and loses carbon dioxid, while the air cells lose oxygen and gain the latter gas.

[Illustration:  Fig. 92.—­Capillary Network of the Air Cells and Origin of the Pulmonary Veins.

  A, small branch of pulmonary artery;
  B, twigs of the pulmonary artery anastomosing to form peripheral network
     of the primitive air cells;
  C, capillary network around the walls of the air sacs;
  D, branches of network converging for form the veinlets of the pulmonary
     veins.
]

The blood thus becomes purified and reinvigorated, and at the same time is changed in color from purple to scarlet, from venous to arterial.  It is now evident that if this interchange is to continue, the air in the cells must be constantly renewed, its oxygen restored, and its excess of carbon dioxid removed.  Otherwise the process just described would be reversed, making the blood still more unfit to nourish the tissues, and more poisonous to them than before.

215.  Change in the Air in Breathing.  The air which we exhale during respiration differs in several important particulars from the air we inhale.  Both contain chiefly the three gases, though in different quantities, as the following table shows.

Oxygen.     Nitrogen.    Carbon Dioxid. 
Inspired air contains      20.81       79.15         .04
Expired air contains       16.03       79.58        4.38

That is, expired air contains about five per cent less oxygen and five per cent more carbon dioxid than inspired air.

The temperature of expired air is variable, but generally is higher than that of inspired air, it having been in contact with the warm air passages.  It is also loaded with aqueous vapor, imparted to it like the heat, not in the depth of the lungs, but in the upper air passages.

Expired air contains, besides carbon dioxid, various impurities, many of an unknown nature, and all in small amounts.  When the expired air is condensed in a cold receiver, the aqueous product is found to contain organic matter, which, from the presence of micro-organisms, introduced in the inspired air, is apt to putrefy rapidly.  Some of these organic substances are probably poisonous, either so in themselves, as produced in some manner in the breathing apparatus, or poisonous as being the products of decomposition.  For it is known that various animal substances give rise, by decomposition, to distinct poisonous products known as ptomaines.  It is possible that some of the constituents of the expired air are of an allied nature.  See under “Bacteria” (Chapter XIV).